There are many different instances where photoimagable compositions are utilized in various industrial processes. In one particular process a photoimagable composition is utilized as a solder mask by applying the composition to the underlying printed circuit board. Thereafter photolithographic techniques are employed to reveal various underlying structures on the board while masking others so that solder may be applied by various solder applying processes to the exposed structures. During the solder applying process the solder will adhere to the exposed underlying components and be prevented from adhering where the remaining material operates as a solder mask.
It is necessary that the solder mask material be formulated such that it can be applied by the appropriate methods; for example curtain coating, a preferred method, requires certain rheological properties for effective coating. Further, the solder mask must have the properties of providing efficient transmission of the light or other exposing radiation so as to photolyze the photoinitiator through whatever thickness of material is applied. Also, of course, if the material is to be used as a solder mask, the solder mask must possess appropriate physical and chemical properties to withstand the application of the solder material without significant deterioration, degradation or loss of adhesion to the underlying substrate and maintain its coverage over the portions of the board wherein solder is to be masked. If it is to be used for other purposes, such as an etch resist, other properties may be required.
There have been many prior art proposals for different photoimagable compositions including many that use epoxies. Examples of these are found in the following U.S. Pat. Nos. 4,279,985; 4,548,890; 4,351,708; 4,138,255; 4,069,055; 4,250,053; 4,058,401; 4,659,649; 4,544,623; 4,684,671; 4,624,912; 4,175,963; 4,081,276; 4,693,961; and 4,442,197. All of these patents show various resins and photoinitiators for use in photoimageable compositions many of which are useful as solder masks. However none of them teach or suggest the specific composition of the present invention.
Good photopatternable formulations which permit high resolution fine line photopatterning have been disclosed in the parent application Ser. No. 07/318,536, which issued as U. S. Pat. No. 5,026,624. However, these formulations may not be suitable for use on epoxy-glass laminate circuit boards, which are cured with dicyandiamide. It has been found that certain coating formulations demonstrate poor image resolution when applied to epoxy-glass laminates cured with dicyandiamide. It is desirable to have a liquid photoimagable coating which may be used as a solder mask, containing epoxy resin cured with photosensitive cationic initiators which demonstrate good image resolution when applied to epoxy-glass laminates, cured with dicyaniamide.